We have performed microwave measurements on superconducting hot-isostatically- pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave surface resistance decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 nm to 110 nm through surface-polishing and ion-milling. The surface resistance was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low surface resistance (~0.8 mOhms at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.
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